A system of rat conceptus culture during the period of organogenesis when the embryos are most susceptible to anomalous development has been established showing growth and advanced differentiation in vitro are comparable to those in vivo during the same periods of gestation. Rat embryos removed from the mother on pregnancy days 8-12 are cultured for 1-3 days in homologous rat serum in the pertidishes or in roller bottles with appropriate O2 atmosphere. Utilizing these systems of in vitro culture, we propose to survey effects of 4 classes of model toxins/teratogens on in vitro embryo development: alkylating agents (TEM, nitrogen mustard, cyclophosphamide), antimetabolites (BUdR, 5-FU, methotrexate), hydrocarbons (DMBA, Benz(a)pyrene), and halogenated agents (TCDD, PCB). The relative ability of directly or indirectly acting toxins/teratogens to induce anomalous development will be evaluated by adding these substances at different concentrations in medium with or without a microsomal activation system. The conceptuses will also be cultured with extracts of preincubation of teratogen, microsomes and cofactor in culture medium (serum). The normal and abnormal development of conceptuses will be assessed by gross morphology, histology, and DNA/protein measurements in embryos, yolk sacs and placental tissues. The morphological and biochemical changes induced by these agents in rat embryos in vitro will be compared with those in vivo. Transport of these agents from the medium to the growing embryos in vitro, metabolism of these agents by different compartments of conceptuses, and specific localization of these substances on organ primordia of embryos will be determined. Processes associated with microsomal bioactivation, including cofactor, cytochrome and O2 requirements of indirectly acting agents, when conceptuses are cultured in vitro with a microsomal activating system will be evaluated. The comparative ability of bioactivation of teratogens by microsomes from different sources, such as liver, placenta and uterine decidual tissues, or hepatic microsomes from TCDD/phenobarbital treated rats will be examined. The study of bioactivation processes of teratogens with human placental microsomes in the rat embryo culture system will be attempted.